Relating to electrical power assisted steering

ABSTRACT

An electric power assisted steering system is disclosed in which a motor ( 1 ) is adapted to provide, an assistance torque to an output-shaft ( 12 ) through a worm gear ( 13 ) and wheel gear assembly ( 11 ) and a biasing means ( 14 ) is provided which biases the shaft carrying the worm gear ( 13 ) into engagement with the shaft carrying the wheel gear ( 11 ). The biasing means ( 14 ) may comprise a spring member provided between the gearbox housing and the bearing ( 9 ) supporting the free end of the worm gear shift, or a torsion spring adapted to act upon the bearing assembly.

[0001] This invention relates to improvement in gear assemblies, and inparticular to electrical power assisted steering assemblies whichincorporate a worm and wheel gear assembly for transferring torque froman electric motor to a steering column or output shaft operativelyconnected thereto.

[0002] It is known to provide a power steering system for a vehiclecomprising an electric motor having a stator and a rotor, an input shaftoperatively connected to the rotor and adapted to rotate therewith, anoutput shaft associated with a steering column, and a gearbox adapted totransfer torque from the input shaft to the output shaft in response toa measure of torque in the output shaft produced by a torque sensor. Themotor is typically operated to apply an increasing torque to the outputshaft as the measured torque increases, thus applying an assistancetorque which helps to steer the vehicle.

[0003] In a simple arrangement, the input shaft defines a worm gear, andthe output shaft is provided with a wheel gear which is adapted to meshwith the worm gear. Whilst such a system is relatively effective, thereexists a problem with noise and vibration due to incorrect meshingbetween the worm and gear wheel. This incorrect meshing may arise due tomanufacturing tolerances, thermal changes in dimensions, distortion dueto torsional loads and wear during service.

[0004] According to a first aspect, the invention provides an electricpower assisted steering system comprising a housing, an electric motorfixed relative to the housing having a stator and a rotor, an inputshaft operatively connected to the rotor, an output shaft operativelyconnected to a steering column, and a torque sensor adapted to producean output signal indicative of the torque in the output shaft, the motorbeing adapted to apply a torque to the output shaft dependent upon theoutput signal from the torque sensor through a worm gear provided on theinput shaft which is adapted to mesh with a wheel gear on the outputshaft, the steering system being characterised by further comprising afirst bearing means which supports the input shaft relative to thehousing at its end distal from the motor and a resilient biasing meansadapted to act upon the first bearing means to bias the input shafttowards the wheel gear.

[0005] Preferably, the input shaft is biased in a tilting movement whichis centred at a second bearing means which supports the input shaftrelative to the housing at its end adjacent to the motor.

[0006] The biasing means may be adapted to apply a sufficient biasingforce to the first bearing means to maintain a fully meshed engagementbetween the teeth of the worm gear and the teeth of the wheel gear overa predetermined range of torque values carried by the wheel gear. Thishelps to prevent gear rattle when driving straight ahead or on roughroads by ensuring both sides of the engaging teeth on the worm and wheelare in contact at substantially all times over this range of torques.Because the arrangement increases quiescent friction in the gearbox itis important to maintain control of the force applied by the biasingmeans over the full range of the input shaft that is required. Thereforethe biasing means must have a low spring rate.

[0007] The provision of the biasing means allows a controlled biasingforce to be applied whilst permitting sufficient tilting movement of theinput shaft to compensate for changes in dimensions due to manufacturingvariations and temperature changes etc. The maximum torque value up towhich the fully meshed engagement is effective is carefully chosen (bycompromise) to avoid excessive friction.

[0008] The biasing means may comprise a resilient spring of any typeadapted to act between a portion of the housing and the first bearingmeans.

[0009] In some configurations, it is preferred that the resilient springcomprises a leaf spring which may be attached to the housing at a firstend and act upon the first bearing means at its second end. This mayengage the first bearing means at the opposite side of the input shaftto the wheel gear so as to bias the worm into contact with the wheelgear.

[0010] The leaf spring may be provided outside of the housing and thesecond end of the leaf spring may pass through an opening in the housingto engage with the first bearing means. The second end of the leafspring may carry a seal which seals with the opening through which itpasses.

[0011] The input shaft may be directly connected to the motor rotor. Itmay extend continuously through the rotor.

[0012] The input shaft may be operatively connected to the rotor througha flexible coupling which allows the worm to tilt without movement ofthe rotor.

[0013] The flexible coupling may comprise a resilient element, forexample of rubber. The motor rotor may be adapted to apply a drive forceto the resilient element through one or more circumferentially spacedradially extending surfaces of the resilient element. The resilientelement may in turn be adapted to apply a drive force to the input shaftthrough one or more other circumferentially spaced radially extendingsurfaces of the element. The element may comprise a spider shape havinga multiple of arms presenting a number of radially extending,circumferentially spaced drive surfaces.

[0014] Where the input shaft is connected to the motor rotor by aflexible coupling, a first compression means may be provided between thehousing and the first bearing means at the end of the input shaft distalfrom the motor which applies a compressive force onto the input shaft tobias it towards the motor rotor. It may comprise a coil spring. Itsfunction is to prevent noise and vibration due to axial free play in thesecond bearing means. In its compressed state, there should be freespace between adjacent coils in order to avoid frictional resistance totees tilting motion of the input shaft.

[0015] A second compression means (such as a coil spring) may also beprovided between the end of the input shaft adjacent the motor rotor andthe motor rotor. This may be provided in a cup formed on an end of theinput shaft whilst a pin extending about the rotational axis of themotor rotor and forming a part of the rotor projects into the cup toengage the spring.

[0016] Thus, whilst the first compression means biases the secondbearing means through the flexible coupling, the second compressionmeans biases the rotor directly through the pin. There must be adifference between the forces provided respectively by the first andsecond compression means which is sufficient to bias the second bearingmeans in the direction of the input shaft axis by the desired amount.

[0017] In an alternative, the biasing means may comprise an annularO-ring provided between the first bearing means and the housing. TheO-ring may be of rubber, and may contact an outer circumference of thefirst bearing means and a portion of the housing.

[0018] The first bearing means may therefore move relative to thehousing against a resistant force applied by the O-ring as at least partof the O-ring is compressed. In such an arrangement, the wheeled gear orthe worm gear may deliberately be made slightly oversize relative to thedimensions which are calculated according to the distance between theirrespective shaft axes. This ensures the O-ring is always under a smallamount of residual compression.

[0019] In an alternative to an O-ring, the resilient biasing means maycomprise a resilient element which is accommodated between the firstbearing means and the housing, such as a rubber spacer block. Theelement may be disposed between the first bearing means and the housing,opposite to the side of the input shaft which engages the gear wheel.The biasing means may act in compression or tension.

[0020] The second bearing means may comprise a ball bearing which isadapted to prevent radial and axial movement of the input shaft relativeto the housing whilst permitting tilting movement of the input shaftagainst the bias force provided by the biasing means.

[0021] The second bearing means may be selected to comprise a hightolerance ball bearing assembly which is adapted by virtue of the shapeof the groove in which the balls are located to substantially preventany radial displacement of the input shaft relative to the housing as itpasses through the second bearing whilst permitting the input shaft topivot about a point on its axis which passes through a point in theplane of the second bearing means.

[0022] In a preferred arrangement, the housing defines a first portionand a second portion, the first portion comprising a housing for theinput shaft having at least one pair of opposed walls, and having anopening in each wall into which the first and second bearing means arerespectively provided, and the second portion comprising a housing forat least part of the output shaft having at least one pair of opposedwalls, an opening being provided in each wall for receiving one or morebearings adapted to secure the output shaft relative to the housing. Theoutput shaft is preferably mounted orthogonal to the input shaft andsubstantially prevented from moving radially relative to the housing.

[0023] A plastic lining portion may be provided around a circumferentialouter face of the first bearing means which prevents contact between thefirst bearing means and the housing at excessive displacement. Thishelps to eliminate vibration noise due to metal-metal contact betweenthe first bearing means and the housing.

[0024] The first opening defined in the first portion of the housing(which receives the first bearing means) may comprise an elongated slotthrough which the input shaft passes having semicircular end portionsand a central pair of parallel sides. The spacing between the parallelsides may be substantially the same as the outer diameter of the firs,bearing means. The radius of the semicircular end portions of the slotmay be substantially the same as the outer radius of the first bearingmeans. Thus, the first bearing means may be adapted to move axiallyalong the slot but may be prevented from moving radially perpendicularto the slot.

[0025] The first opening of the first portion may comprise an annularbore having an inner surface with a diameter greater than the diameterof the first bearing means. The outer surface of the first bearing meansmay thus be spaced from the inner wall. The O-ring element forming thebiasing means may be accommodated in this space.

[0026] A groove may be provided around a circumference of the inner wallwhich locates the O-ring in a predetermined position relative to thewall, the depth of the groove being less then the radial thickness ofthe O-ring element. In this case, the first bearing means may bedisplaced from its rest position against the biasing force provided bythe O-ring through a distance equal to the difference between the O-ringradial thickness and the depth of the groove. Thereafter, the firstbearing means is prevented from further radial displacement within theopening as it engages the inner wall of the first opening.

[0027] In a yet further alternative arrangement, the biasing means; maycomprise a torsion bar having a first end acting upon the first bearingmeans and a second end fixed relative to a portion of the housing sothat the torsion bar applies a biasing force against the first bearingmeans.

[0028] The torsion bar may comprise a substantially U-shaped elongaterod having a terminal end portion on a first end of the roll which isbent through an angle of approximately ninety degrees relative to theremaining part of the first end and relative to the centre portion ofthe rod to engage with a portion of the first bearing means. Preferably,the terminal end portion acts directly on an outer surface of the firstbearing means opposed to the wheel gear of the output shaft by pas singthrough an opening channel in the housing extending radially away fromthe inner wall of the first opening of the first portion of the housing.

[0029] The central portion of the torsion bar may be secured to aportion of the housing through one or more clamps or shackles.

[0030] The second end of the torsion bar may rest upon an end face of athreaded bolt which engages with the housing. Rotation of the boltwithin the threaded bore displaces the second end of the torsion barrelative to the housing. As the first end is engaged with the firstbearing means this acts to increase or decrease torsion in the bar in aknown manner, in turn to alter the biasing force applied to the firstbearing means (i.e. for use when setting up).

[0031] In a preferred arrangement, a terminal portion of the second endof the torsion bar is bent through approximately ninety degrees relativeto the remaining part of the second end portion and engages within arecess in the end face of the bolt. This provides a positive locationfor the second end portion.

[0032] In yet a further alternative arrangement, where space in thevehicle permits, the biasing means may comprise a coil spring leavingits axis substantially perpendicular to that of the wormshaft. The coilspring could be installed in a hole in the housing. A first end of thespring could apply force to the outer race of the first bearing meansvia a formed end of the spring or via a separate component placedbetween the spring and the first bearing means. A closure plug or plateat the end of the hole distal from the bearing means would provide asupport to the coil spring and a means of sealing.

[0033] In a most preferred arrangement, the terminal portion of thesecond end of the torsion bar engages with a recess in the housing. Thisrenders the arrangement non-adjustable and tamper proof which ispreferable for production versions.

[0034] In a refinement, where the biasing means comprises an O-ring sealacting between the first bearing means and the housing, the centre axisof the O-ring may be offset relative to the central axis of the inputshaft. This provides a different relationship between biasing force anddisplacement of the bearing means compared to the case where the centralaxes of the O-ring and shaft coincide. It is preferred that the axis ofthe O-ring is closer to the wheel gear than that of the input shaftwhere is passes through the first bearing means.

[0035] To further refine the relationship between the biasing force anddisplacement of the first bearing means, the shape of the O-ring groove(where provided) relative to the cross-section of the O-ring may bechosen so that the compressed portion of the O-ring just completelyfills the groove at a predetermined displacement corresponding to apredetermined biasing force, whereafter the rate of increase of biasingforce with full displacement is significantly greater than the rate ofincrease of biasing force with displacement at displacements below thepredetermined displacement. When the O-ring is in its normal positioncorresponding to zero torque on the gear wheel, the O-ring may thereforeonly partially fill the groove at this point.

[0036] According to a second aspect, the invention provides an electricpower assisted steering system comprising a housing, an electric motorfixed relative to the housing having a stator and a rotor, and inputshaft operatively connected to the rotor, an output shaft operativelyconnected to a steering column, and a torque sensor adapted to producean output signal indicative of the torque in the output shaft, the motorbeing adapted to apply a torque to the output shaft dependent upon theoutput signal from the torque sensor through a worm gear provided on theinput shaft which is adapted to mesh with a wheel gear on the outputshaft, the steering system being characterised in that the input shaftis operatively connected to the motor rotor by a flexible coupling.

[0037] By providing a flexible coupling, it is possible for the inputshaft to tilt relative to the motor rotor. This enables the position ofthe worm relative to the wheel to be adjusted to remove gear rattlewithout having to move the rotor.

[0038] Preferably, the flexible coupling comprises a resilient element.It may be a rubber element.

[0039] The element may have a plurality of substantially radiallyextending drive faces. One or more of the drive faces may co-operatewith one or more radial drive faces defined on the rotor. One or more ofthe drive faces may co-operate with drive faces defined on the inputshaft. The motor rotor thus can apply torque to the input shaft throughthese faces.

[0040] Preferably, the element comprises a spider having a plurality ofradially extending arms defining the drive surfaces.

[0041] The input shaft may have a cup formed on its end adjacent themotor rotor. A pin located along the axis of rotation of the rotor maybe adapted to be received within the cup. The flexible coupling may beprovided between an end face of the cup and the rotor, perhaps aroundthe pin.

[0042] The cup may be adapted to receive a first resilient biasingelement such as a spring which acts between the end of the pin and thebast of the cup to bias the rotor away from the input shaft.

[0043] A second compression means may be provided which is adapted tobias the input shaft towards the rotor. This may comprise a springlocated between the housing and a bearing means which supports the inputshaft.

[0044] There will now be described three examples of the presentinvention by way of example only. Reference is made to the accompanyingdrawings which include like reference numerals for like parts, of which:

[0045]FIG. 1 is a cut-away partial view of a first embodiment of anelectrical power assisted steering system which incorporates the presentinvention;

[0046]FIG. 2 is a view of a second embodiment of an electricalpower-assisted steering system incorporating the present invention;

[0047]FIG. 3 is an alternate view of the system of FIG. 2 showing theconnection between the torsion bar and a threaded bolt which is fastenedto the housing;

[0048]FIG. 4 is a view of a third embodiment of an electricalpower-assisted steering system incorporating the present invention;

[0049]FIG. 5 is a detail of the second support hub for the motor rotor;

[0050]FIG. 6 is a detail of the flexible spider element; and

[0051]FIG. 7 shows in plan the leaf spring element.

[0052]FIG. 1 is a cut-away view of part of an electrical power-assistedsteering system of the present invention for use in a vehicle.

[0053] A motor 1 for applying assistance torque to an output shaft 12operatively connected to a steering column shaft comprises a stator 2and a rotor 3. The motor is mounted onto a side of a housing 4. One endof an input shaft 5 which is splined to an end of the rotor extendsthrough an opening 8 into an inner cavity 6 of the housing. The otherend of the input shaft passes through an opening 7 on an opposite sideof the housing to the opening 8, and a first bearing means 9 and secondbearing means 10 located in the openings 7 and opening 8 respectivelysupport the input shaft relative to the housing.

[0054] The input shaft 5 carries a worm gear 13 between the two bearingmeans which is adapted to engage with a toothed wheel 11 provided on theoutput shaft 12 where it passes through the housing. Bearings (notshown) support the output shaft 12 relative to the housing 4 orthogonalto the axis of the input shaft 5 so that the worm gear and wheel gearare meshed.

[0055] In use, an output from a torque sensor (not shown) adapted tomeasure the torque in the output shaft 12 (or a steering shaftoperatively connected thereto) is passed to an electronic control unit(ECU) in turn to produce a motor drive signal which controls the torqueproduced by the motor 1. The motor 1 then transfers torque through themotor rotor 3 to the input shaft 5 and onto the output shaft 12 toprovide assistance to aid a driver of the vehicle.

[0056] Each of the bearing means 9, 10 comprises a ball bearing orroller bearing cartridge having an inner bearing race which co-operateswith the input shaft and an outer bearing race spaced around the innerrace, bearings being provided therebetween. Any well known bearingassembly can be used subject to meeting the requirements for toleranceand load bearing set out by the designer.

[0057] The second bearing means 10 is secured to the housing 4 and actsas a pivot about which the input shaft 5 may tilt. It preventssubstantially all radial movement of the shaft 5 as it passes throughthe bearing 10.

[0058] The first bearing means 9 is spaced from the housing 4 by aresilient biasing means 14 adapted to bias the input shaft 5 towards thegear wheel 12 of the output shaft 11.

[0059] The second (and larger) bearing means 10 therefore reactstangential forces being applied to the gear wheel by the worm, as wellas radial forces (i.e. at right angles to the axis of the worm) due tothe helix angle and pressure angle of the teeth.

[0060] The first bearing assembly 9 is constrained axially relative tothe housing 4 (as described hereinafter) but is free to move radiallyagainst the biasing force applied by the biasing means 14.

[0061] The biasing means 14 acts to bias the worm into mesh with thegearwheel via an elastic medium and to allow it to adopt a fully meshedcondition (i.e. where there is no clearance between the flanks on eithersides of the engaging worm and gearwheel teeth) for the range ofgearwheel size and position variations (due to manufacturingtolerances), temperatures and states of tooth wear. As shown in FIG. 1,the biasing means comprises an O-ring which locates in a groove 15having a square cross section.

[0062] It is required to maintain this fully meshed condition for arange of torque values, measured at the gear wheel, (for example up to4N-m in one application) in order to prevent gear rattle when drivingaround the straight ahead on rough roads. A force of 20N needs to beapplied to the worm, radially with respect to the gearwheel in order tomaintain full meshing at 4 N-m gearwheel torque. When a higher torque isapplied, then the worm will move away from the gearwheel and, clearancewill occur at the sides of the teeth which are not transmitting thetorque. The maximum torque rating of the gear system shown in FIG. 1 is42 N-m.

[0063] Experiments have shown that the range of dimensional backlashvariation due to tolerances, temperature and wear that may arise, if abiasing means was not incorporated, is typically around 0.150 mm. Tocompensate for this, a range of radial displacements of the worm,relative to the gearwheel, is needed which is approximately 2× thebacklash variation (because of the 14 degree pressure angle); i.e. 0.300mm total (or +/−0.150 mm from the nominal worm axis position). Thisrange of displacements is provided by allowing the wormshaft to pivotaround the larger ball bearing, nominally moving in a vertical plane,and to bias the movement towards the gearwheel by means of the 3 mm wideO-ring acting on the outer race of the smaller ball bearing,

[0064] In the particular design shown in FIG. 1 the ratio of the lengthsfrom the engaging centre of the worm to the centres of the respectivebearings means that a force of 20×48[48+38.5] N (=11N) should be appliedby the O-ring. This should ideally be maintained over a range ofdisplacements, from the nominal worm axis position, of +/−0.270 mm(=+/−0.150×[48 +38.5)/48] mm). However, this is not practical becausethe force must change with movement in the case of such a simple elasticmedium. As a compromise, the invention achieves a force range of approx.17 to 27 N over the 0.540 mm (i.e. +/−0.270 mm) total displacementrange. This is achieved by positioning the centre of the O-ring grooveto be offset relative to the nominal position of the worm axis. Theamount of that offset will depend on the Force vs. Displacementcharacteristic of the O-ring, which will act as a non-linear springwhose rate will increase as the worm is forced further away from thegearwheel. Once the limit of the above working displacement range isexceeded, in the direction away from the gearwheel, then the resistanceof the O-ring should rise very rapidly to prevent excessive wormshaftmovement at high torques.

[0065] An absolute limit of travel is provided (for example 0.500 mmfrom the nominal axis) by virtue of the fact the bore in the housing forthe 22 mm diameter bearing is machined to 23 mm. To avoid metal-to-metalimpact noise, the force of the O-ring at 0.500 mm off centredisplacement of the small bearing may be chosen to be at least 150 N.Thins will require a very fast rising spring rate between 0.300 and0.500 mm displacement. This can be achieved by tailoring the preciseshape of the O-ring groove, in relation to the diameter of the O-ring'scross section, such that the O-ring material just fills the groove atthe exact point at which the spring rate is required to rise steeply.The hardness of the rubber is another parameter that can be optimised.

[0066] Note that it is important to limit the meshing force which occursat the lower torques because it induces a significant amount ofquiescent friction into the operation of the gearbox and this isdetrimental form efficiency and good road feel. A meshing force of 20 Nwill create 0.4 N of friction as measured at the gearwheel. The maximumacceptable is typically around 0.5 N.

[0067] To enable it to act as a pivot centre for the wormshaft, at leastover the small angular displacements involved (+/−0.18 degrees), thelarger bearing can be specified as a “C3” clearance grade (i.e. with themaximum standard clearance choice). This will allow the bearing to runwith the required misalignment without excessive friction and wear. Toprevent it from rattling due to the sporadic gearbox torque reversalsthat occur when driving straight ahead on rough roads, it bearing may beaxially pre-loaded at 90N. The pre-load can be applied via the shaft bypreloading the smaller bearing's outer race by means of a compressedcoil spring 16.

[0068] The connection between the wormshaft 5 and the motor rotor 3 isvia clearance spline engagement in which a small leaf spring is used tolaterally load the male spline relative to the female spline and removeany torsional free play in the motor drive. This arrangement permits asmall amount of bending compliance between the wormshaft and the motorrotor and hence allows the desired displacement of the wormshaft.

[0069] An alternate embodiment is shown in FIGS. 2 and 3 and wherepossible, like reference numerals have been used to those provided onFIG. 1 as many components are identical.

[0070] The second embodiment differs from the first embodiment in so faras the biasing means comprises a torsion bar 100 acting upon an outersurface of the first bearing means instead of an O-ring seal.

[0071] The torsion bar 100 comprises an elongate bar bent into anelongated U-shape. Each end of the bar is further bent over thoughapproximately ninety degrees.

[0072] The central portion 101 of the bar 100 is clamped onto a portionof the housing 4 through a bush which allows the bar to rotate about itsaxis. One end 102 of the bar acts upon the bearing means whilst theother 103 acts upon a bolt 104 threadably engaged with the housing 4.Rotation of the bolt 104 displaces the associated end of the bar,inducing torsion in the bar as the other end is prevented from moving bythe bearing means. Thus, the biasing force can be adjusted by rotatingthe bolt.

[0073] Of course, a bolt is not essential, and many other ways ofvarying the biasing force can be employed. For example, shims may beinserted between the end of the torsion bar and either the bearing meansor the housing. Alternative spring types are also envisaged within thescope of the invention.

[0074] The skilled man will therefore understand that the presentinvention lies, in at least one aspect, in the provision of a biasingmeans (which biases the input shaft towards the output shaft with adesired Displacement Biasing Force relationship so as to at leastpartially prevent gear rattle.

[0075] A third embodiment of an electrical power assisted steeringsystem of the present invention is illustrated in FIG. 4.

[0076] This system differs from the previous two systems in that theworm gear is provided on an input shaft 5 which is isolated from themotor rotor 3 by a flexible coupling. It is also biased against thewheel gear 11 using a leaf spring 200 arranged to act upon the firstbearing means 9 supporting the end of the shaft 5 opposite the motor.

[0077] The motor rotor 3 is cylindrical and is supported at each end bya respective hub 30, 31. A first hub 30 comprises a supporting framewith a bearing located on its axis of rotation. The bearing is a slidingfit over a centred stud 32 which forms a part of the motor housing 4 andwhich is located on the central axis of rotation of the rotor. The studthus passes through the bearing to provide support for the rotor at theend of the motor opposite to the gearbox.

[0078] The second hub 31 comprises a radial supporting frame and anintegral centrally located pin 33. The pin 33 projects outwardly fromthe rotor 3 towards the gearbox and is accommodated in a cup 51 formedon the end of the input shaft 5 to ensure approximate axial alignment ofthe rotor and the input shaft 5.

[0079] Between the second hub 31 and the cup 51 is a flexible coupling52 comprising a rubber spider with eight identical, circumferentiallyspaced arms 53 defining sixteen radial drive surfaces 54.

[0080] As can be seen in FIG. 6, the second hub 31 has four drive teethor dogs 31 a projecting towards the gearbox which engage betweencorresponding arms of the flexible spider. The cup 51 on the worm gearshaft is similarly provided with four teeth or dogs which extend axiallytowards the rotor and engage the remaining drive surfaces between thearms of the spider. Thus, drive from the motor rotor is coupled to theworm shaft through the spider.

[0081] Providing the flexible coupling, which can be seen in more detailin FIGS. 5 and 7 of the accompanying drawings allows the input shaft 5to move without the need for corresponding movement of the motor rotor3, enhancing the operating life of the motor.

[0082] A small coil spring 55 is provided within the cup which actsthrough a spacer onto the end of the pin of the second hub to bias themotor rotor away from the gearbox. A second spring 56 is providedbetween the gearbox housing and the first bearing means to bias theinput shaft 5 towards the rotor.

[0083] The leaf spring 200 can be seen in more detail in FIG. 7 of theaccompanying drawings. It comprises a substantially planar resilientelement that is bolted 202 at one end onto the motor housing with theplane of the spring parallel to the axis of the input shaft.

[0084] The free end 201 of the leaf spring 200 is bent through ninetydegrees and passes through an orifice in the housing to engage the firstbearing means 9. This applies a force to the input shaft directedtowards the wheel gear. An over moulded seal on the end 201 of the leafspring co-operates with the walls of the orifice to seal the orifice. Inaddition, a cover plate 300 is provided which prevents access to theleaf spring unless the cover is removed.

[0085] The first bearing means supporting the free end of the worm gearshaft is located within a plastics guide that is an interference fitwithin the housing. The guide is oversize by 0.76 mm in respect ofmovement of the first bearing means radially towards and away from thewheel gear but is a close tolerance fit in the orthogonal (horizontal)direction to restrain movement of the bearing in that direction. Theguide thus allows only one degree of freedom of movement of the firstbearing means.

1. An electric power assisted steering system comprising a housing 4, anelectric motor 1 fixed relative to the housing 4 having a stator 2 and arotor 3, an input shaft 5 operatively connected to the rotor 3, anoutput shaft 12 operatively connected to a steering column, and a torquesensor adapted to produce an output signal indicative of the torque inthe output shaft 12, the motor 1 being adapted to apply a torques to theoutput shaft 12 dependent upon the output signal from the torque sensorthrough a worm gear 13 provided on the input shaft 5 which is adapted tomesh with a wheel gear 11 operatively connected to the output shaft 12,the steering system being characterised by further comprising a firstbearing means 9 which supports the input shaft 5 relative to the housing4 at its end distal from the motor 1 and a resilient biasing means 14,200 adapted to act upon the first bearing means 9 to bias the inputshaft 5 towards the wheel gear
 11. 2. An electric power assistedsteering system according to claim 1 in which the input shaft is biasedvia a tilting movement which is centred at a second bearing means whichsupports the input shaft relative to the housing at its end adjacent tothe motor.
 3. An electric power assisted steering system according toclaim 1 in which the biasing means 14, 200 is adapted to apply asufficient biasing force to the first bearing means 9 to maintain afully meshed engagement between the teeth of the worm gear 13 and theteeth of the wheel gear 11 over a predetermined range of torque valuescarried by the wheel gear.
 4. An electric power assisted steering systemaccording to claim 1 or claim 2 in which the biasing means comprises aresilient spring 200 adapted to act between a portion of the housing 4and the first biasing means
 9. 5. An electric power assisted steeringsystem according to claim 4 in which the resilient spring 200 comprisesa leaf spring which is attached to the housing at a first end and actsupon the first bearing means 9 at its second end.
 6. An electric powerassisted steering system according to claim 5 in which the leaf spring200 is provided outside of the housing 4 and the second end of the leafspring 200 passes through an opening in the housing 4 to engage with thefirst bearing means
 9. 7. An electric power assisted steering systemaccording to claim 4 in which the second end of the leaf spring 200 isadapted to carry a seal which seals with the opening through which itpasses.
 8. An electric power assisted steering system according to anyone of claims 1 to 4 in which the biasing means comprises a torsion barspring having a first end acting against the first bearing means and asecond end fixed relative to a position of the housing.
 9. An electricpower assisted steering system according to claim 8 in which the secondend of the torsion bar rests upon an end face of an adjustment meanswhich engages with the housing.
 10. An electric power assisted steeringsystem according to any one of claims 1 to 4 in which the biasing meanscomprises an annular O-ring provided between the first bearing means andthe housing.
 11. An electric power assisted steering system according toany one of claims 1 to 4 in which the biasing means comprises a coilspring having its axis substantially perpendicular to that of thewormshaft.
 12. An electric power assisted steering system accordingclaim 11 in which the coil spring is provided in a hole in the housingand a closure plug or plate is provided at the end of the hole distalfrom the first bearing means which provides a support for the coilspring.
 13. An electric power assisted steering system according to anypreceding claim in which the input shaft 5 extends continuously throughthe rotor 3 and is sufficiently flexible to allow the biasing means todeflect the worm by the required amount.
 14. An electric power assistedsteering system according to any one of claims 1 to 12 in which theinput shaft 5 is operatively connected to the rotor 3 through a flexiblecoupling 52 which allows the worm to move without movement of the rotor3.
 15. An electric power assisted steering system according to claim 14in which the flexible coupling 52 comprises a resilient element and themotor rotor 3 is adapted to apply a drive force to the resilient element52 through one or more circumferentially spaced radially extendingsurfaces 54 of the resilient element.
 16. An electric power assistedsteering system according to claim 15 in which the resilient element 52is adapted to apply a drive force to the input shaft 5 through one ormore circumferentially spaced radially extending surfaces 54 of theelement.
 17. An electric power assisted steering system according toclaim 14, 15 or 16 in which a first compression means is providedbetween the housing and the first bearing means at the end of the inputshaft distal from the motor which applies a compressive force onto theinput shaft to bias it towards the motor rotor.
 18. An electric powerassisted steering system according to claim 17 in which the firstcompression means comprises a coil spring.
 19. An electric powerassisted steering system according to any one of claims 14 to 18 inwhich a second compression means (such as a coil spring) may also beprovided between the end of the input shaft adjacent the motor rotor andthe motor rotor.
 20. An electric power assisted steering systemaccording to claim 19 in which a cup is formed on an end of the inputshaft 5 whilst a pin extending about the rotational axis of the motorrotor and forming a part of the rotor projects into the cup to engagethe second compression means
 55. 21. An electric power assisted steeringsystem according to any one of claims 3 to 20 when dependent upon claim2 in which the housing defines a first portion and a second portion, thefirst portion comprising a housing for the input shaft having at leastone pair of opposed walls, and having an opening in each wall into whichthe first and second bearing means 9, 10 are respectively provided, andthe second portion comprising a housing for at least part of the outputshaft 12 having at least one pair of opposed walls, an opening beingprovided in each wall for receiving one or more bearings adapted tosecure the output shaft relative to the housing.
 22. An electric powerassisted steering system according to any preceding claim in which aplastic lining portion is provided around a circumferential outer faceof the first bearing means which prevents contact between the firstbearing means and the housing at excessive displacements.
 23. Anelectric power assisted steering system according to any preceding claimin which an opening defined in the first portion of the housing whichreceives the first bearing means 9 comprises an elongated slot throughwhich the input shaft 5 passes having semicircular end portions and acentral pair of parallel sides whereby the first bearing means 9 isadapted to move axially along the slot but prevented from movingradially perpendicular to the slot.
 24. An electric power assistedsteering system comprising, a housing, an electric motor fixed relativeto the housing having a stator and a rotor, and input shaft operativelyconnected to the rotor, an output shaft operatively connected to asteering column, and a torque sensor adapted to produce an output signalindicative of the torque in the output shaft, the motor being adapted toapply a torque to the output shaft dependent upon the output signal fromthe torque sensor through a worm gear provided on the input shaft whichis adapted to mesh with a wheel gear on the output shaft, the steeringsystem being characterised in that the input shaft is operativelyconnected to the motor rotor by a flexible coupling.
 25. An electricpower assisted steering system according to claim 24 in which theflexible coupling comprises a resilient element.
 26. An electric powerassisted steering system according to claim 25 in which the element hasa plurality of substantially radially extending drive faces, one or moreof the drive faces co-operating with one or more radial drive facesdefined on the rotor and one or more of the drive faces co-operatingwith drive faces defined on the input shaft.
 27. An electric powerassisted steering system according to claim 26 in which the elementcomprises a spider having a plurality of radially extending armsdefining the drive surfaces.
 28. An electric power assisted steeringsystem according to any one of claims 24 to 27 in which the input shafthas a cup formed on its end adjacent the motor rotor and a pin locatedalong the axis of rotation of the rotor is adapted to be received withinthe cup